The bottom threads of the spark tube have a thread-lock compound on them from the factory. That prevents the tubes from accidently turning out when the valve cover top nuts are loosened, and seals the threads from oil. As a result, given the tubes are very difficult to loosen and remove from the head, the sealant doesn't typically wear out and cause an oil leak.
Most times when oil gets inside a spark plug tube, it does not originate at the bottom from the threads going into the head. It originates from the top, where the rubber seals in the underside of the valve cover get hard and don't seal against the tubes well. If you never had the spark plug tubes out, and if you never changed the seals for the spark plug tubes that are installed in the underside of the valve cover, I would most certainly change them first to stop your oil leak into the tubes. Most people damage the tubes when they try to remove them, not knowing the threads at the bottom most likely is not the source of their oil leaks in the first place.